In this study we extend research on online collaboration by examining motivation to do work on the crowdsoucing service Amazon Mechanical Turk (MTurk). We address a challenge to many existing studies of motivation in online contexts: they are based on survey self-reports, which are susceptible to effects such as social desirability bias. In addition we investigate a second challenge to the extant research on motivation in the context of MTurk: a failure to examine potential differences between MTurk workers (Turkers) from different parts of the world, especially those from the US and India, MTurk's two largest worker groups. Using a survey technique called the list experiment, we observe distinct profiles of motivation and patterns of social desirability effects among Turkers in the US and India. Among US Turkers, we find that social desirability encourages over-reporting of each of four motivating factors we examined. The over-reporting was particularly large in the case of money as a motivator. In contrast, among Turkers in India we find a more complex pattern of social desirability effects, with workers under-reporting "killing time" and "fun" as motivations, and drastically over-reporting "sense of purpose." We conclude by discussing these results and proposing implications for future research and design.